FIG. 6 illustrates a measuring system 61 with a diagram describing the triangulation principle for measuring a distance of an object. See for instance Fuji Electric Review, Vol. 68, No. 7, (1995), pp 415-420. Rays from an object 55 are focused on optical sensor arrays 53, 54 through lenses 51, 52 as object images 56, 57. Points G, H are the crossing points of the parallel rays passing the centers C, D of the lenses 51, 52 from an infinite distance, i.e., the optical axes of the lenses 51, 52, and optical sensor arrays 53, 54. The distance between the points G and H is represented by B. The distance between the optical sensor array 53 or 54 and the lens 51 or 52 is represented by fe. The displacements of object images 56 and 57 from optical axes 58 and 59 are represented by X1 and X2, respectively. The sum of X1 and X2 is represented by X. A line 60 extends between points A and E, and extends perpendicular to the line CD.
Since the triangles ACE and CFG are proportional to each other and the triangles AED and DHI are proportional to each other, the distance d of object 55 can be obtained from the following equation (1):d=B·fe/(X1+X2)=B·fe/X  (1),where the sum X is the relative displacement of two object images 56 and 57 from the reference in which the object images 56 and 57 are at the cross points of the optical axes 58, 59 of the lenses 51, 52 and the optical sensor arrays 53, 54. Since the distances B and fe are known constants, the distance d can be obtained by detecting the sum X.
Referring to FIG. 7, which is an exploded perspective view showing the structure of a range finder employing the above-described triangulation principle, the range finder includes an auto-focus IC (hereinafter referred to as an “AFIC”) 76, a pair of lenses 71, 72 and a pair of optical sensor arrays 81, 82 and a shield box 73. An IR (infrared ray) cut filter 74 is inserted if necessary. The AFIC 76 and the lenses 71, 72 are fixed to the shield box 73.
FIG. 8(a) is a top plan view of a semiconductor chip sealed in the AFIC. FIG. 8(b) is a top plan view of optical sensor arrays. FIG. 8(c) is a top plan view of an optical sensor. A pair of optical sensor arrays 81, 82, amplifier circuits 83a, 83b, 84a, 84b arranged around optical sensor arrays 81, 82, address circuits 85a, 85b, 86a, 86b, and reference voltage circuits 87a, 87b are integrated into a semiconductor chip 80. In FIG. 8(a), bonding pad arrays 88a and 88b are shown. The amplifier circuits 83a, 83b, 84a, 84b include therein integrator circuit arrays that integrate the currents of the optical sensors. The optical sensor arrays 81 and 82 each include many optical sensors 91a and 91b, respectively. Each optical sensor 91a or 91b includes a light receiving area (light receiving section) 98 and a pickup electrode 92. The light receiving area 98 is formed of a photodiode or a phototransistor. The photodiode or the photo-transistor is formed by implanting ions or by diffusing ions into the surface portion of a semiconductor substrate.
JP PA 2002-077670 discloses a method for correcting the deviation in the positioning of lenses 71, 72 and the optical sensor arrays 81, 82 by widening the regions of optical sensors 91a, 91b in the direction that is perpendicular to the direction in which a pair of the optical sensor arrays 81, 82 are disposed, namely spaced side by side or longitudinally aligned.
Now the main circuit and the structure of a conventional range finder and a camera that mounts the conventional range finder thereon follows. FIG. 9 is a block circuit diagram of an optical sensor circuit that constitutes the conventional range finder. The optical sensor circuit 100 includes an optical sensor 101 and an integrator circuit 102 that integrates the photocurrent flowing through optical sensor 101 to convert the photocurrent to a voltage. The optical sensor 101 is a photoelectric converting device such as a photodiode and a phototransistor. The integrator circuit 102 is a circuit that includes an operational amplifier 103 and a capacitor 104 connected parallel to each other. A positive voltage is applied to the cathode of a photodiode, which forms the optical sensor 101 and a negative voltage to the anode of the photodiode. When light impinges onto the photodiode, photocurrent flows. The integrator circuit 102 integrates the photocurrent and outputs the voltage corresponding to the integrated photocurrent as a sensor output 105.
Referring to FIG. 10, which is a block diagram showing a conventional optical sensor array, an optical sensor circuit array 130 is formed by aligning the optical sensor circuits 100, each formed of the optical sensor 101 and the integrator circuit 102, into a line. In other words, the optical sensor circuit array 130 is formed by aligning the optical sensors 101a, 101b, 101c, etc., into an optical sensor array 110, by aligning the integrator circuits 102a, 102b, 102c, etc., into an integrator circuit array 120, and by arranging the optical sensor arrays 110 and the integrator circuit arrays 120 parallel to each other.
FIG. 11 is a diagram showing the main portion of the conventional range finder. The conventional range finder includes a pair (110a and 110b) of optical sensor arrays 110 shown in FIG. 10 and arranged on the right and left hand sides in FIG. 11, a pair (120a and 120b) of the integrator circuit arrays 120 shown in FIG. 10 and arranged on the right and left hand sides in FIG. 11, an output circuit 132, into which the sensor output signals from the integrator circuit arrays 120a and 120b are input, and a control circuit 131. A pair of optical sensor circuit arrays 130a,130b, formed of a pair of optical sensor arrays 110a, 110b, and a pair of integrator circuit arrays 120a, 120b are integrated in a semiconductor chip (AFIC). A pair of range finding lenses (lenses 71 and 72) are arranged right above the pair of optical sensor arrays 110a and 110b. 
FIG. 12 is a diagram showing a conventional camera mounting thereon the range finder shown in FIG. 11. The conventional camera 140 mounts thereon a range finder 142 that is used as an automatic focusing device. Since it is usually difficult to mount the range finder 142 on one side of an image pickup lens 141 due to the limitations caused by the layouts of the constituent elements of the camera 140, the range finder 142 is mounted right above the image pickup lens 141 or diagonally above the image pickup lens 141. The object image is focused on a photosensitive plane by measuring the distance of the object with the range finder 142 and by moving the image pickup lens 141 on the camera 140 based on the measured data. When a range finder is formed in combination with an optical system for focusing the object image on optical sensors aligned into a line, the vertical field of view is limited to one direction and a certain angle of visibility determined by the optical system.
Since parallax forms between the vertical field of view E on the pair of optical sensor arrays 110a, 110b through a range finding lens 143 and the vertical field of view F on a photosensitive plane 144, such as a film through image pickup lens 141, parallax regions 145 are formed. If the object is in any of the parallax regions 145, it will be impossible for the pair of optical sensor arrays 110a, 110b in the range finder to detect the object and to measure the object distance accurately. Therefore, it will be impossible to focus the object image accurately onto the photosensitive plane 144.
To obviate the problem described above, a multi-line sensor that includes many lines of optical sensors is employed. See for instance JP PA 2001-350081, p. 17 and FIG. 12 thereof. FIG. 14 illustrates a main portion of a range finder employing such a multi-line sensor. The multi-line sensor is formed of multiple pairs of optical sensor circuit arrays 130 shown in FIG. 11 arranged perpendicularly to the direction in which each pair of optical sensor circuit arrays 130 are arranged. In the multi-line sensor, integrator circuit arrays 155a, 155b are arranged between optical sensor arrays 151a, 151b arranged side by side on an upper line, and optical sensor arrays 152a, 152b arranged side by side on a middle line. In the same manner, integrator circuit arrays are arranged between the optical sensor arrays arranged side by side on the middle line and the optical sensor arrays arranged side by side on a lower line.
However, when the multiple sensor arrays are used, the regions occupied by the integrator circuit arrays are not the regions in which any optical sensor is disposed. Therefore, wide spaces are formed between the adjacent pairs of optical sensor arrays 151a, 151b and 152a, 152b, and between the adjacent pairs of optical sensor arrays 152a, 152b and 153a, 153b as shown in FIG. 15. A plurality of parallax regions 162 are formed, where the field of view P for the photosensitive plane 161 seen through the image pickup lens 141 and the fields of view L, M, N for the adjacent pairs of optical sensor arrays 151a, 151b; 152a, 152b; and 153a, 153b seen through the range finding lens 143 do not overlap each other. In other words, parallax regions still exist. Due to the parallax regions, it becomes impossible to measure the object distance accurately. Therefore, it becomes impossible to focus the object image on photosensitive plane 161 accurately. Since the integrator circuits are disposed for the respective optical sensors, the number of wiring is also increased. Due to the increasing number of wiring, the area occupied by the wiring and the area occupied by the integrator circuit arrays are widened. Therefore, the chip size is increased.
A range finder having a small chip size and capable of correcting the parallax problem accurately is described, for instance, in JP PA 2002-360788 (unpublished patent application), which published as JP 2004-191739 on 8 Jul. 2004. FIG. 16 is a block circuit diagram of an optical sensor circuit illustrated in that reference. The block circuit diagram described in FIG. 16 corresponds to the optical sensor circuit described in FIG. 9. The optical sensor circuit 210 includes multiple optical sensors (three optical sensors 201, 202, and 203 in the figure) for constituting multiple optical sensor arrays, selection switches (MOSFETs 204, 205, 206) for selecting the optical sensors 201, 202, 203, and an integrator circuit 207 for integrating the current from the optical sensors 201, 202, 203. The optical sensors 201, 202, 203 are, for example, photodiodes. The integrator circuit 207 is, for example, a circuit in which an operational amplifier 208 and a capacitor 209 are connected parallel to each other. When light impinges onto the photodiode, photocurrent flows. The integrator circuit 207 integrates the photocurrent and outputs the voltage corresponding to the integral photocurrent as a sensor output.
FIG. 17 is a diagram showing the layout of the optical sensor circuit arrays constituting the range finder shown in FIG. 16. FIG. 17 corresponds to FIG. 10 showing the layout of a conventional optical sensor array. The optical sensor circuits 210 shown in FIG. 16 are aligned to form an optical sensor circuit array 220. The optical sensor circuit array 220 includes three optical sensor arrays 221, 222, 223 and an integrator circuit array 224 arranged such that a multi-line sensor has a very narrow spacing between the lines (sensor arrays). The number of the optical sensor arrays (lines) is not limited to 3.
Since only the selection switches, which are MOSFETs 204a, 204b, 204c, etc., MOSFETs 205a, 205b, 205c, etc., and MOSFETs 206a, 206b, 206c, etc., are disposed in the spaces between the lines, it is possible to narrow the line spacing. The upper optical sensor array 221 is formed of optical sensors 201a, 201b, 201c, etc., the middle optical sensor array 222 is formed of optical sensors 202a, 202b, 202c, etc., and the lower optical sensor array 223 is formed of optical sensors 203a, 203b, 203c, etc. When the MOSFETs 204a, 204b, 204c, etc. are switched on, the photocurrents from optical sensors 201a, 201b, 201c, etc., flow to the integrator circuits 207a, 207b, 207c, etc. constituting the integrator circuit array 224. The integrator circuits 207a, 207b, 207c, etc. integrate the respective photocurrents and output the voltages corresponding to the respective integral photocurrents as sensor output signals.
FIG. 18 is a block diagram schematically showing the main portion of the range finder using the optical sensors shown in FIG. 16. The range finding lenses and such constituent elements are not illustrated in FIG. 18. The range finder in FIG. 18 includes 3 lines of sensor arrays: an upper pair of optical sensor arrays 221a, 221b, a middle pair of optical sensor arrays 222a, 222b, and a lower pair of optical sensor arrays 223a, 223b. A pair of integrator circuit arrays 224a, 224b is disposed for the 3 lines of sensor arrays. As described above, the optical sensors in the optical sensor arrays are connected via the respective small selection switches, i.e., MOSFETs, (not shown) to the respective integrator circuits constituting the integrator circuit arrays. The integrator circuits integrate the photocurrents from the optical sensors in the selected optical sensor arrays, convert the integral photocurrents to voltages, and output the respective converted voltages as sensor output signals. The signals for switching on and off the MOSFETs are transmitted from a control circuit 225 arranged in the central part of the range finder. The sensor output signals from integrator circuits 224 are input to an output circuit 226. The output signal corresponding to the sensor output signals from the integrator circuits 224 is transmitted from the output circuit 226 to the image pickup optical system including the image pickup lenses. In FIG. 18, the wiring among the sensors, the integrator circuits, the control circuit, and the output circuit have been omitted.
FIG. 19 is a diagram describing the relations between the object distances measured by the range finder shown in FIG. 18 and the fields of view. Now explanations will be made on measuring the distances of objects 229 on a measuring axis 228 spaced apart for a predetermined distance from and extending parallel to an optical axis 227 connecting the center of a range finding lens 230 and the center of the lower pair of optical sensor arrays 223a, 223b, fixed such that optical sensor arrays 223a, 223b are in a light receiving area receiving the light from an infinite point.
The distance of an object 229a in a far range is measurable, since the object 229a is in the field of view A of the lower pair of optical sensor arrays 223a, 223b. The distance of an object 229b in a middle range is measurable, since object 229b is in the field of view B of the middle pair of optical sensor arrays 222a, 222b. The distance of an object 229c in a close range is measurable, since object 229c is in the field of view C of the upper pair of optical sensor arrays 221a, 221b. By selecting the optical sensor array pair suitable for light receiving depending on the range, in which the object is located in the field of view of the optical sensor arrays independently of the object distance and, therefore, accurate distance measurement can be facilitated.
The above-identified reference, JP PA 2002-360788, describes 3 lines of sensor array group exhibiting a parallax correction function. This configuration is useful for increasing the number of lines to 13 as shown in FIG. 20 to widen the field of view vertically as shown in FIG. 21 so that a multi-point range finder can be obtained. The range finder as shown in FIG. 20 facilitates two-dimensional image recognition.
FIG. 22 is a cross sectional view of the main portion of the range finder shown in FIG. 20. In FIG. 22, the optical sensors constituting two lines in the B section surrounded by the dotted lines in FIG. 20 are shown. The B section corresponds to the C section surrounded by the dotted lines in FIG. 16. The reference numeral 251 designates one optical sensor in a line 1, which is a first optical sensor. The reference numeral 252 designates one optical sensor in a line 2, which is a second optical sensor. A p-type well region 322 is formed in the surface portion of an n-type substrate 321. A first n-type region 323 and a second n-type region 324 are formed in the surface portion of the p-type well region 322 such that the first and second n-type regions 323 and 324 are spaced apart from each other. A third n-type region 325 is formed adjacent to the first n-type region 323 and a fourth n-type region 326 is formed adjacent to the second n-type region 324. A first gate electrode 330 is formed above a portion of the p-type well region 322 extending between the first and third n-type regions 323 and 325 with a gate insulator film 329 interposed between the first gate electrode 330 and the extending portion. A second gate electrode 332 is formed above the other extending portion of the p-type well region 322 extending between the second and fourth n-type regions 324 and 326 with gate insulator film 329 interposed between the second gate electrode 332 and the other extended portion. The first n-type region 323 and p-type well region 322 constitute a first optical sensor 251. The second n-type region 324 and p-type well region 322 constitute a second optical sensor 252. A first MOSFET 253 is formed of the first n-type region 323, the third n-type region 325, and the first gate electrode 330. A second MOSFET 254 is formed of the second n-type region 324, the fourth n-type region 325, and the second gate electrode 332.
The MOSFETs 253 and 254 are selection switches for connecting the optical sensors 251 and 252 to an integrator circuit 255. By switching on the MOSFETs 253 and 254, that is, by switching on the selection switches, the optical sensors 251, 252 are connected to an integrator circuit 255. The optical sensors in this state are referred to as the “selected optical sensors.” On the other hand, by switching off the MOSFETs 253 and 254, the optical sensors 251 and 252 are disconnected from the integrator circuit 255. The optical sensors in this state are referred to as the “unselected optical sensors.”
Many first optical sensors are aligned to form a first optical sensor array. A first optical sensor line (line 1) is formed of a pair of first optical sensor arrays. Many second optical sensors are aligned to form a second optical sensor array. A second optical sensor line (line 2) is formed of a pair of second optical sensor arrays. Both the third and fourth n-type regions 325 and 326 are connected to the input of the integrator circuit 255. In this manner, the range finder as shown in FIG. 20, having 13 optical sensor lines and capable of finding the ranges of multiple points, is formed. The range finder includes as many integrator circuits as there are the optical sensors (448 in FIG. 21) included in a pair of optical sensor arrays (one optical sensor line).
A positive voltage, e.g., around 1.6 V, is applied from the input side of integrator circuit 255 to the third and fourth n-type regions 325 and 326. The p-type well region 322 is grounded. A positive supply voltage, e.g., around 3.3 V, is applied to the n-type substrate 321. An ON-signal is fed to the first gate electrode 330, to which the selection signal 1 is to be supplied, to form a channel 335 in the first MOSFET 253. Pairs of an electron 338 and a hole 339 are generated by the light that has impinged onto the first optical sensor 251 (the selected optical sensor) in the pair of first optical sensor arrays in the first optical sensor line (line 1). Photoelectron current 340 flows from the first n-type region 323 to the integrator circuit 255 via the channel 335 and the third n-type region 325, raising the sensor output voltage. The photo-hole current 341 flows to the ground.
When an ON-signal is fed to the first gate electrode 330, to which the selection signal 1 is to be supplied, an OFF-signal is fed to second gate electrode 332, to which the selection signal 2 is to be supplied. Since the second MOSFET 254 is OFF, no channel is formed in the second MOSFET 254. Since no channel is formed, the second n-type region 324, which is the second optical sensor 252 (the unselected optical sensor), is in the floating state. In other words, the potential of the second n-type region 324 is floating. If pairs of electron 338 and hole 339 are caused by the light that has impinged onto the second optical sensor 252 in the floating state, photo-hole current 341 will flow to the ground. However, the photoelectron current 340 will flow into the fourth n-type region 326 biased at a positive potential and further to the integrator circuit 255, causing noises. In short, the noises are formed by the electrons generated by the light that has impinged onto the unselected optical sensor.
The noises are high especially in the multi-point range finder that includes many optical sensor lines, since many optical sensors are left unselected. When the range finder includes 13 optical sensor lines, one of the optical sensor lines is selected and the rest of the optical sensor lines (12 lines) are unselected. Due to many unselected optical sensor lines, the ratio of the noise photoelectron currents from the unselected optical sensors in the 12 optical sensor lines to the normal photoelectron currents flowing to the integrator circuits is as high as 40%, making it hard to conduct accurate range finding.
In view of the foregoing, there still remains a need for a range finder that reduces the noises caused by the unselected optical sensors and measuring the object distance accurately. The present invention addresses this need.